The OPT -1 trial, whose clinical component is sponsored by the National Institutes of Health, is testing the hypothesis that parenteral or oral insulin administration will delay the development of type 1 diabetes amongst cytoplasmic ICA positive relatives of patients with diabetes. The trial represents the largest evaluation of first degree relatives ever undertaken with more than 80,000 relatives screened, and more than 100,000 sera samples analyzed for cytoplasmic ICA, GAO65 and ICA512 (IA-2) autoantibodies and 3,000 analyzed for insulin autoantibodies (micro-lAA assay) under the aegis of this grant. A supplement to this grant has allowed limited one time "staging" of a subset of relatives with "biochemical" autoantibodies but lacking cytoplasmic ICA. Preliminary results indicate: One half of cytoplasmic ICA positive individuals lack all biochemical autoantibodies and have a low probability of being eligible on Staging for the OPT -1 trial. Approximately one fourth of relatives expressing multiple autoantibodies are cytoplasmic ICA negative. Expression of "biochemical" autoantibodies is related to relationship to proband and HLA status. A new micro-insulin autoantibody assay (micro-lAA) has a higher correlation with multiple autoantibody expression compared to the standard anti-insulin assay. A subset of DaB 1 *0602 positive individuals progress to overt diabetes. We believe from the preliminary data that "biochemical" autoantibody determination will be essential for future trials of type 1 A diabetes prevention. Nevertheless specific predictive paradigms cannot yet be precisely specified (too few determinations of micro-IAA (insulin autoantibody), and too little follow up of ICA negative biochemical autoantibody positive relatives). It is now essential to take advantage of the cohorts already assembled and new screenees over the next several years to obtain adequate prospective metabolic and immunologic evaluation. Newer assays (including epitope specific assays and subclass specific assays) combined with the unique OPT clinical study, will be important both for the practical development of "biochemical" autoantibody prediction and understanding of the "natural history" and pathogenesis of type 1A diabetes and response to "insulin" therapy. The current grant seeks support to continue to determine GAO65 and ICA512 (IA-2) autoantibodies on new samples and to apply new assays, to measure with the recently developed high-throughput insulin autoantibody assay all samples, to prospectively evaluate on an annual basis cytoplasmic ICA negative, biochemical autoantibody positive individuals, to analyze genetic determinants of autoantibody phenotype and metabolic progression, as well as further analyze the wealth of accumulating data.